A display unit such as an LCD (Liquid Crystal Display) is mounted to a mobile terminal device such as a mobile phone (see Japanese Laid-open Patent Publication No. 2008-113101). FIG. 12 is an external perspective front view of an existing mobile phone. FIG. 12 illustrates one example of the existing mobile phone. The existing mobile phone 9 illustrated in FIG. 12 has a movable side housing 90 having a display unit. The movable side housing 90 has an LCD module 91 and a case 92. In the example illustrated in FIG. 12, the LCD module 91 is mounted to the case 92.
With reference to FIG. 13, the following will describe an existing method of mounting the LCD module 91 to the case 92. FIG. 13 is a view for illustrating one example of an existing method of mounting the LCD module 91 to the case 92. As illustrated in FIG. 13, the LCD module 91 and the case 92 have engaging latches for engaging both of them. The engaging latch of the LCD module 91 may be referred to as “LCD side engaging latch”. The engaging latch of the case 92 may be referred to as “case side engaging latch”. In the example illustrated in FIG. 13, the LCD module 91 has an LCD side engaging latch 91a. The case 92 has case side engaging latches 92a and 92b. The LCD module 91 also has an LCD side engaging latch at a position corresponding to the case side engaging latch 92b. The LCD module 91 has a plurality of LCD side engaging latches. The case 92 has a plurality of case side engaging latches.
Such an LCD module 91 is mounted to the case 92 by being pressed into the case 92. For example, in the example illustrated in FIG. 13, the LCD module 91 is put on an upper surface of the case 92, and then press-fitted into the case 92 in a direction D91 from a surface of the LCD module 91. The LCD module 91 is mounted to the case 92 by the LCD side engaging latch 91a moving past the case side engaging latch 92a such that the LCD side engaging latch 91a engages the case side engaging latch 92a. 
There is a proposal for a method of sliding a sub-display unit, which is a second display unit of a mobile phone, into a case called a guide part, thereby mounting the sub-display unit to the case (see Japanese Laid-open Patent Publication No. 2007-208338). It is thought that an operation for mounting the sub-display unit to the case is facilitated by this method. The sub-display unit may be referred to as sub-display or back display.
However, an existing method using engaging latches has a problem that a display unit may be broken. FIG. 14 is a cross-sectional view taken along the line III-III, as seen from an arrow B1 in FIG. 12. The LCD module 91 illustrated in FIG. 12 has the LCD side engaging latch 91a at a portion indicated by the line III-III, and the case 92 illustrated in FIG. 12 has the case side engaging latch 92a at a portion indicated by the line III-III. FIG. 14 illustrates a state where the LCD module 91 is press-fitted into the case 92.
As illustrated in FIG. 14, when the LCD module 91 is press-fitted into the case 92, the LCD side engaging latch 91a is pressed and moved inwardly by the case side engaging latch 92a. When moving past the case side engaging latch 92a, the LCD side engaging latch 91a receives inward stress. At this time, the LCD module 91 is deformed into an arcuate shape as illustrated FIG. 14, and thus may be broken. When the LCD side engaging latch 91a is pressed and moved inwardly, an edge portion E91 of the LCD module 91 receives stress, and thus is likely to be broken.
It is also conceived that an LCD module 91, which is a main display unit, is mounted to a movable side housing by using the above method of sliding the sub-display unit to be mounted to the case. However, the method of sliding the LCD module 91 to be mounted to the case 92 has a problem that a mobile phone is increased in size. FIGS. 15A and 15B are cross-sectional views taken along the line II-II, as seen from an arrow B2 in FIG. 12. As illustrated in FIG. 15A, the case 92 receives the LCD module 91, a board 93, a GPS (Global Positioning System) frame 94, a panel 97, and the like.
When the LCD module 91 is slid to be mounted to the case 92, a slide region for sliding the LCD module 91 is provided in the case 92 as illustrated in FIG. 15B. In order to secure the slide region, the movable side housing 90 (illustrated in FIG. 12) is lengthened in the slide direction. In the field of mobile phones, reduction of housing in size is a common challenge. Thus, it is not desirable that the movable side housing 90 is increased in size.